Vapor compression systems, such as heat pumps, refrigeration and air-conditioning systems, are widely used in industrial and residential applications. The introduction of variable speed compressors and variable position valves to the vapor compression cycle has greatly improved the flexibility of the operation of such systems. It is possible to use these new components to improve the efficiency of vapor compression systems by controlling the components correctly.
For example, a speed of the compressor can be adjusted to modulate a flow rate of a refrigerant. The speed of an evaporator fan and a condenser fan can be varied to alter heat transfer coefficients between air and heat exchangers. The change in an expansion valve opening can directly influence a pressure drop between the high-pressure side and the low-pressure side of the vapor compression system, which, in turn, affects the flow rate of the refrigerant as well as superheat at the corresponding evaporator outlet. The possibilities to control operations of components vapor compression systems offer opportunities for improving both energy efficiency and reliability of the systems.
The operation cycle of the vapor compression system starts from compressing the refrigerant by the compressor into a high-temperature, high-pressure vapor state, after which the refrigerant flows into the condenser. Since the air flowing over the condenser coils is cooler than the refrigerant, the refrigerant cools down to form a high-pressure, low-temperature liquid upon exiting the condenser. The refrigerant then passes through a throttling valve that creates a large pressure drop, so that the pressure of the refrigerant after leaving the valve decreases. The low-pressure refrigerant boils at a much lower temperature, so the air passing over the evaporator coils heats up the refrigerant. Thus, the air is cooled down, and the low-pressure liquid refrigerant is converted to a low-pressure vapor. This low-pressure, low-temperature vapor then enters the compressor, and the operation cycle of the vapor compression system is repeated.
The operation of the typical vapor compression system is affected by a set of environmental parameters, such as thermal load on the system as well as air temperatures at an evaporator and a condenser. Some of these environmental parameters, such as the indoor temperature, have a desired value, i.e., a setpoint, for users of the vapor compression system. For example, the setpoint can be one variable, e.g., the indoor temperature. Also, the setpoint can be a set of multiple variables, such as the temperature and relative humidity of the indoor air.
The operation of the vapor compression system is also defined by a set of thermodynamic parameters of the refrigerant, such as evaporating pressure Pe, the amount of superheat at the evaporator outlet (SH), condensing pressure Pc, and the amount of subcooling at the condenser outlet (SC). The setpoint can be provided for both the environmental and the thermodynamic parameters.
Typically, the operation of the vapor compression system is regulated by a control system. The control system converts the setpoint and the thermodynamic parameters to a set of control inputs, which control the various components of the vapor compression system to reach and maintain the thermodynamic parameters and the setpoint at a specified level. The set of control inputs can include a speed of a compressor, a position of an expansion valve, and the speed of fans in both the evaporator and the condenser.
Accordingly, it is desired to determine the set of control inputs that optimizes a performance of the vapor compression system. A number of methods for controlling operations of the vapor compression system are disclosed in the art. However, there is a need in the art for improving those methods.
For example, one method [disclosed in U.S. Pat. No. 7,246,500] determines optimal energy consumption by comparison of operation of the vapor compression system controlled by modulating the condenser fan speed. However, large changes in the cooling load might result in suboptimal energy consumption because the system does not apply sufficient adjustments to the condenser fan speed to find the optimal value of this fan speed.
Another method for controlling a vapor compression system [disclosed in U.S. Pat. No. 5,735,134] considers the possibility of sudden change in environmental or thermal load requirements, monitors the vapor compression system in real-time, and determines, based on these actual real-time measurements, a set of parameters to enable the system to operate at maximum coefficient of performance.
Hence, this method determines the set of control inputs every time upon the change in environmental or thermal load requirements. However, such determination is time consuming, and requires substantial real time computational resources. However, some applications require the minimization of computational complexity while determining the optimal set of control inputs in real time during the operation of the vapor compression system.
Another method [disclosed in U.S. Pat. No. 7,076,962] first determines amount of heat flow across an evaporator or a condenser. Next, the amount of heat flow is used to determine the set of optimal control inputs. As the amount of heat flow is directly related to the operation of the vapor compression system, its determination is difficult to avoid. However, there are applications in which it is desired to determine the optimal set of control inputs without determining the amount of heat that the vapor compression system needs to transfer in accordance with a desired setpoint.
Yet another method [disclosed in JP 2000-257941] reduces energy consumption of cold water or hot-water in the air conditioner by measuring the room temperature with a thermometer and retrieving a value of a valve opening from a valve opening table using the room temperature as an index. However, conventional vapor compression systems typically have number of different components, including but not limited to the valve, which need to be controlled concurrently. Moreover, this method determines the valve opening based on outside environment conditions only, which is not always optimal.